Liquid crystal display devices are used in watches and clocks, electric calculators, various types of measuring equipment, panels for automobiles, word processors, electronic message pads, printers, computers, televisions, etc. The liquid crystal displays take advantage of optical anisotropy and anisotropy of dielectric constant which liquid crystalline compounds possess. Known display systems for this purpose include twisted nematic (TN), super-twisted nematic (STN), dynamic scattering (DS), guest-host (G-H), DAP, ferroelectric liquid crystal (FLC) display systems. Drive systems, for these display systems, known in the art include static drive, time-sharing drive, active-matrix drive, and double-channel drive systems.
Various properties are required of the liquid crystal materials depending upon display systems and drive systems. However, 1) a wide temperature range of liquid crystal phase and 2) low viscosity are important properties required in common to all the systems. Properties necessary for meeting the requirement 1) include a high upper limit temperature of the nematic phase and a melting point low enough to prevent phase separation such as crystallization in a low temperature region.
The properties of liquid crystalline compounds used in these various liquid crystal display devices vary depending upon applications of the liquid crystal display devices. However, good stability against external environment factors, such as moisture, air, heat, and light, are required of the all the liquid crystalline compounds used for these purposes. Further, these liquid crystalline compounds are required to exhibit a liquid crystal phase in an as wide as possible temperature range around room temperature.
A liquid crystal composition for use in liquid crystal display devices comprises several to twenty-odd liquid crystalline compounds in order to develop optimal properties required of individual display devices. For this purpose, good compatibility with other liquid crystalline compounds, particularly good compatibility with other liquid crystalline compounds at a low temperature due to an ever-increasing demand for use of the display device under various environments, is required of the liquid crystalline compounds.
Liquid crystal compositions for use in the STN drive system are required to have steep threshold properties from the viewpoint of realizing a high image quality. The steepness is a function of the elastic constant ratio K33/K11, and it is known that the steepness of the threshold properties increases with increasing the elastic constant ratio of liquid crystalline compounds used in the liquid crystal composition (F. Leenhouts et al., Proceedings of the Japan Display, 388 (1986)).
The following compounds having an alkenyl site are known to have a large elastic constant ratio K33/K11: the following compounds (a) described in M. Schadt et al., Mol. Cryst. Liq. Cryst., 122 (1985) and Japanese Patent Laid-Open No. 83136/1986: ##STR2## and the following compounds (b), containing a fluorine atoms, described in Japanese Patent Application No. 92740/1994: ##STR3##
When the above alkenyl compounds were mixed in an amount of 15% by weight with a liquid crystal composition comprising
24% of 4-(4-propylcyclohexyl)benzonitrile, PA1 36% of 4-(4-pentylcyclohexyl)benzonitrile, PA1 25% of 4-(4-heptylcyclohexyl)benzonitrile, and PA1 15% of 4-(4-pentylphenyl)benzonitrile, PA1 1-methyl-1-ethenyl(isopropenyl), 1-ethyl-1-ethenyl, 1-propyl-1-ethenyl, 1-isopropyl-1-ethenyl, 1-butyl-1-ethenyl, 1-isobutyl-1-ethenyl, 1-pentyl-1-ethenyl, 1-isopentyl-1-ethenyl, 1-(2-methylbutyl)-1-ethenyl, 1-hexyl-1-ethenyl, 1-isohexyl-1-ethenyl, 1-(2-methylpentyl)-1-ethenyl, 1-(3-methylpentyl)-1-ethenyl, 1-heptyl-1-ethenyl, 1-octyl-1-ethenyl, 1-nonyl-1-ethenyl, 1-decyl-1-ethenyl, 2-methyl-2-propenyl, 2-ethyl-2-propenyl, 2-propyl-2-propenyl, 2-isopropyl-2-propenyl, 2-butyl-2-propenyl, 2-isobutyl-2-propenyl, 2-pentyl-2-propenyl, 2-isopentyl-2-propenyl, PA1 3-methyl-3-butenyl, 3-ethyl-3-butenyl, 3-propyl-3-butenyl, 3-butyl-3-butenyl, 3-pentyl-3-butenyl, 3-hexyl-3-butenyl, 3-heptyl-3-butenyl, 3-octyl-3-butenyl, 3-nonyl-3-butenyl, 3-decyl-3-butenyl, 4-methyl-4-pentenyl, 4-ethyl-4-pentenyl, 4-propyl-4-pentenyl, 4-butyl-4-pentenyl, 4-pentyl-4-pentenyl, 4-hexyl-4-pentenyl, 4-heptyl-4-pentenyl, 4-octyl-4-pentenyl, 4-nonyl-4-pentenyl, 4-decyl-4-pentenyl, 5-methyl-5-hexenyl, 5-ethyl-5-hexenyl, 5-butyl-5-hexenyl, 5-pentyl-5-hexenyl, 5-hexyl-5-hexenyl, 5-heptyl-5-hexenyl, 5-octyl-5-hexenyl, 5-nonyl-5-hexenyl, and 5-decyl-5-hexenyl groups.
and properties were measured, it was found that all the above compounds had a relatively large elastic constant ratio K33/K11 and all the liquid crystal compositions using these compounds had favorable steepness. The upper limit of the elastic constant ratio K33/K11 of a liquid crystalline compound having an alkenyl site on its side chain is that noted above. However, an ever-increasing demand for an improvement in display ability of liquid crystal display devices has led to a demand for compounds having more steep threshold properties. In other words, liquid crystalline compounds having a larger elastic constant ratio K33/K11 have been desired in the art. That is, the provision of a novel skeleton other than alkenyl compounds has been desired in the art.
Compounds (c) having a 1,3-butadienyl group, represented by the following structural formula, described in Japanese Patent Laid-Open No. 286873/1993 are also known to have a high elastic constant ratio K33/K11: ##STR4##
These compounds, however, do not have a satisfactorily high elastic constant ratio K33/K11 and, since the 1,3-butadienyl group has a conjugated diene site, chemically very unstable, making it impossible to use them in liquid crystal compositions for practical purpose.
That is, the development of liquid crystalline compounds, for STN, possessing a very large elastic constant ratio K33/K11, high chemical stability, and good compatibility with other liquid crystalline compounds has been desired in the art.
Liquid crystal compositions designed for active-matrix liquid crystal displays with an integrated non-linear element incorporated thereinto for switching individual pixels, particularly, for TFT, should have large positive anisotropy of dielectric constant and, in addition, very high specific resistance (high voltage retention), good UV stability, and high compatibility with other liquid crystalline compounds at a low temperature.
The active-matrix liquid crystal displays are suitable particularly for displays for televisions, advanced information displays for computers, and advanced information displays in automobiles and airplanes. However, when liquid crystalline compounds or liquid crystalline compositions not having very high specific resistance (high voltage retention) or good UV stability are used, the contrast decreases with a lowering in electrical resistance in the liquid crystal panel, posing a problem of "afterimage quenching."
High electrical resistance of the liquid crystal composition is a very important factor which determines the service life, particularly in the case of low-voltage drive. For this reason, very high specific resistance (high voltage retention) and good UV stability are very important properties required of liquid crystalline compounds used.
Further, in order to enable use of the liquid crystal composition in a wide temperature range, the liquid crystal composition should have a nematic phase particularly at a low temperature. Therefore, liquid crystal compositions free from the precipitation of a crystal or the development of a smectic phase and having a minimized temperature dependence of viscosity at a low temperature have been desired in the art. For this, that the liquid crystalline compound used has high compatibility with other liquid crystalline compounds at a low temperature is very important.
However, no conventional liquid crystalline compounds, for TFT, which have a combination of large positive anisotropy of dielectric constant, very high specific resistance (high voltage retention), good UV stability, high compatibility with other liquid crystalline compounds at a low temperature, are known in the art.
An object of the present invention is to provide a novel liquid crystalline compound having a large elastic constant ratio, excellent compatibility with other liquid crystalline compound(s) particularly at a low temperature, and chemical stability, or a liquid crystalline compound having a combination of a large positive anisotropy of dielectric constant, a very high specific resistance (a high voltage retention), good UV stability, and high compatibility with other liquid crystalline compound(s) at a low temperature, and a liquid crystal composition comprising the same.